1. The Field of the Invention
The present invention relates generally to plasticized sulfur materials usable in paving binder compositions and methods for manufacturing such compositions. More specifically, the present invention relates to the preparation of a plasticized sulfur composition which can be then added to asphalt and an aggregate. This plasticized sulfur material can be prepared independent of the asphalt into which it is added and can be shipped to locations where asphalt is available for preparation of a modified asphalt-based paving binder. Additionally, for remote locations where asphalt, and more particularly quality asphalt, is not readily available, this plasticized sulfur material can be mixed with asphalt and a fine mineral constituent to create a usable asphalt-based binder that will retain non-flow properties within a broad range of ambient temperatures. The present invention also relates to the methods for producing the plasticized sulfur compositions and the asphalt-based paving binders with these compositions.
2. The Relevant Technology
Paving material usually includes a binder and an aggregate. Although the binder is typically the minority component in paving materials, most of the pavement properties that relate to its longevity and performance depend on the properties of the binder.
The binder component is generally an asphalt-based composition that may include various additives. Asphalt is described as a dark brown to black cementitious material, which has a solid, semisolid or liquid consistency, and in which the predominant constituents are bitumens that occur in nature as such or which are obtained as residue in refining petroleum. Natural deposits in which asphalt occurs within porous rocks are known as rock asphalts or tar sands. Petroleum asphalt is part of the residue that is obtained in the distillation of petroleum. In particular, asphalt cement is petroleum asphalt that is refined to meet specifications for paving, industrial, and special purposes.
The aggregate component of paving material is typically any hard, inert, mineral material that is used for mixing in graduated fragments. The aggregate component may include sand, gravel, crushed stone, coral, and slag.
One of the limitations to the use of asphalt as a binder for paving materials is that it softens and flows within a wide range of ambient temperatures. This limitation makes transporting this type of conventional asphalt-based material difficult and can also give rise to serious environmental problems. Further, because of the asphalt's tendency to soften, even at ambient temperatures, modification of the asphalt by the incorporation of various additives has long been known in the art. Asphalt additives are typically used to render the binder material less flowable at ambient temperatures.
Sulfur is one of such additives that has been incorporated into the binder as a minority binder constituent. Mixing asphalt with sulfur, however, presents a number of problems. In order for the sulfur to effectively modify the asphalt, the sulfur must be effectively plasticized or polymerized. This plasticization may occur when the sulfur is mixed with the hot asphalt. However, problems with the plasticization of the sulfur often result as the liquid sulfur, liquid asphalt and aggregate are mixed.
In certain mixtures, the sulfur and asphalt can separate due to the differences in their respective densities which tend to cause an uneven dispersion of the plasticized sulfur. As a result, the sulfur-depleted portions of the binder then retain the softening and flowing properties of asphalt. Not only does the presence of sulfur-depleted portions of binder diminish the overall effectiveness of the asphalt as a binder, but handling and transporting the binder remains difficult. Chemicals such as dicyclopentidiene and heptane have also been used in an attempt to keep the sulfur homogeneously dispersed in asphalt. Further, crushed limestone has been used for this purpose. However, the use of calcium-based materials leads to the formation of calcium sulfides and polysulfides that are detrimental to the pavement longevity.
Additionally, where liquid sulfur, liquid asphalt and aggregate are mixed simultaneously or nearly simultaneously, even with other components, additional problems with the plasticization of the sulfur can occur. Specifically, where part of the liquid sulfur reacts with the aggregate before being completely plasticized by the asphalt, the nonplasticized sulfur bonds with the aggregate rather than completing its plasticization reaction. This non-plasticized sulfur works to weaken rather than strengthen the overall material strength.
Even where the final sulfur-modified asphalt binder is successfully prepared, this process requires the handling of liquid sulfur on site. The presence of liquid sulfur creates potential environmental and material handling concerns.
It has been long believed that sulfur-rich binders may detrimentally affect the quality and longevity of the paving material. In addition, sulfur has been viewed as a constituent that would unacceptably increase the cost of binder materials to the point of rendering them prohibitively expensive if the amount of sulfur in the binder exceeded a certain limit.
In addition to economic considerations regarding the use of sulfur as an additive in paving binders, the use of asphalt is also related to economic factors. For example, the use of asphalt as the major constituent in paving binders is negatively affected by the often fluctuating petroleum production patterns. Further, limited petroleum supplies may threaten, in the long term, the viability of paving binders in which asphalt is a major constituent. Profitable utilization of petroleum products is another factor that detrimentally affects the use of asphalt as a majority constituent in paving binders. For example, maintaining, renovating and protecting the surfaced highways and streets in the United States requires approximately thirty (30) million tons of asphalt cement annually. Asphalt cement was available in the past at a reasonable cost because asphalt cement is a residue in petroleum refining and certain petroleum refining residues could only be economically utilized for the production of asphalt cement. However, higher percentages of petroleum are utilized nowadays for the production of other more profitable forms of petroleum products. As this trend continues, the price of asphalt cement is expected to increase even under constant demand. This expectation is supported by the evolution of the average price of asphalt cement over the past thirty-two years, a period during which the price has risen from approximately $23/ton in 1968 to approximately $152/ton in 2000 (through February), an increase of about 561%. It is generally recognized, however, that there is currently no economical paving binder that can be substituted for asphalt cement, and that there is no low priced asphalt paving binder that can effectively replace high priced asphalt paving binders.
Therefore, there is a need to be able to provide a solid, pre-plasticized sulfur which can be readily mixed with the asphalt to effectively modify the asphalt. Such a pre-plasticized sulfur modifier can reduce the complexity of binder preparation, eliminate the need for handling liquid sulfur in conjunction with liquid asphalt, and provide increased uniformity of binder without concern that nonplasticized sulfur may weaken the paving material. Further, the ability to transport the solid, pre-plasticized sulfur additive material that is in the form of a smaller sized, non-sticky, non-flowing, and non-melting material facilitates the delivery of the pre-plasticized sulfur additive materials to locations where it can be readily mixed with asphalt to create a material acceptable for the specific project specifications. It is desirable to manufacture a binder additive that incorporates pre-plasticized sulfur and which remains in solid, non-sticky, non-flowing and non-melting form over a wide range of ambient temperatures, so that such binder additive can be conveniently transported over long distances by conventional means of transportation for common solids.
Accordingly, there is also a need for improved paving binders which, while still utilizing asphalt, that provide increased performance over asphalt alone. These paving binders should be readily made by incorporating additives into the asphalt. These additive materials would include the following characteristics. First, these additive materials come ready to use without the need of further reaction or modification. These additive materials can also be manufactured in forms that are non-sticky, non-melting, and non-flowing within a wide range of ambient temperatures at which storage and transport is effectuated. Additive materials with these non-sticky, non-flowing, and non-melting properties can be conveniently transported over long distances while avoiding pollution problems that would derive from the emissions and spills of other forms of binders that soften and flow at ambient temperatures. Second, when utilized, these additive materials should substantially reduce the quantity of asphalt in the final paving binders to reduce petroleum dependency and cost. Third, the additives used in the paving binders should not substantially incorporate constituents that, whether directly or when combined with other binder constituents, are known to detrimentally affect the quality and longevity of the pavement.
It is also desirable to manufacture finished plasticized sulfur plus asphalt-based binder compositions in which the sulfur is a majority component and which can be readily shipped to remote locations because the composition remains in solid, non-sticky, non-flowing, and non-melting form over a wide range of ambient temperatures. This material also should impart improvements to the pavement's performance and strength.